The invention concerns an NMR apparatus comprising an NMR magnet system disposed in a first cryocontainer of a cryostat, and an NMR probe head disposed in a room temperature bore of the cryostat and comprising an RF resonator for receiving NMR signals from a sample under investigation, and with a preamplifier, wherein the first cryocontainer is installed in an evacuated outer jacket and is surrounded by at least one radiation shield and/or a further cryocontainer, wherein a cooling device is provided for cooling the NMR probe head and at least one cryocontainer, the cooling device comprising a compressor-operated cryocooler cold head having several cold stages at different temperature levels, wherein at least one cold stage of the cold head is thermally conductingly connected to a heat-transferring device, and wherein at least one cooling circuit with a refrigerant is disposed between the cooling device and the NMR probe head and is driven by the cryocooler compressor or by a pump via a transfer line which is at least partially thermally insulated.
A device of this type is disclosed in WO 03/023433 and EP 1 560 035. NMR apparatus are used for imaging or spectroscopy. They usually contain superconducting magnets, which must be cooled down to very low temperatures.
Most modern NMR magnet systems are still cooled with liquid cryogens (LN2, LHe). Handling of these cryogens is, however, difficult. They must be refilled at regular time intervals that often require undesired interruption of the measurements. The dependence on liquid cryogens is also problematic if the infrastructure is inadequate such as e.g. in developing countries (India, China, etc.). Future cryogen price increases could render such cooling very expensive.
For this reason, attempts have been made to cool magnet systems directly or indirectly using mechanical cooling apparatus, so-called cryocoolers. One concept has proven to be particularly successful with which one or two liquid cryogens are provided inside the cryostat that are reliquefied after evaporation (due to external heat input) using the cryocooler. This produces magnet systems with no external cryogen loss. There are several variants such as e.g. installation of the cold head of the cryocooler directly into the cryostat, in the vacuum-insulated region of the outer jacket of the cryostat or in a helium atmosphere of a neck tube, which directly connects the helium container to the outer jacket.
Cooling of the RF resonator and preamplifier of an NMR probe head has been practiced for some time. This improves the signal-to-noise ratio, i.e. the resolution of the NMR signal and accelerates the measurements. The NMR probe head is cooled via a gas refrigeration circuit that is connected to a cryocooler. The cold head of the cryocooler and the various components of the gas refrigeration circuit, such as heat exchangers and valves, are in a separate thermally insulated housing which is disposed next to the magnet cryostat. The cryocooler is driven by a compressor, which usually has an input power of approximately 7 kW. Cooling of the RF resonator and preamplifier of an NMR probe head is described in U.S. Pat. No. 5,889,456.
Combination of a cryogen loss-free magnet system with an NMR probe head requires two cooling systems occupying a great deal of space, incurring high acquisition and operating costs and having further disadvantages.
WO 03/023433 (Oxford Instruments Superconductivity) therefore discloses combined cooling of a magnet system and a probe head using only one cooler. One single cryocooler (Gifford-McMahon or pulse tube cooler) is thereby used for cooling the (magnet) cryostat and for cooling a more or less rigidly mounted probe head. The cold head of the cooler is thereby integrated in the cryostat. The first cold stage of the cold head is in contact with a radiation shield of the cryostat, while the second cold stage directly liquefies evaporating helium. A separate helium gas circuit driven via a pump is guided over the cold stages where the gas cools and is liquefied to be subsequently guided to the probe head (with gradient coils and an RF resonator in a separate housing that can be evacuated) and the shim coils (in the cryostat or in the housing of the RF unit that can be evacuated) via a line, preferably inside the cryostat, or in a separate housing that can be evacuated.
This arrangement is, however, relatively complicated and precludes flexible operation, since the components (gas lines, probe head) are partially fixed. The thermodynamic efficiency of the conventional arrangement is very low due to the long transfer lines, the heat input via the circulating pump and the contacts between the cooling circuit and the radiation shield at very low temperatures. It is also doubtful whether the refrigeration capacity achieved with current cryocoolers is sufficient to cool the cryostat and the probe head in the manner proposed. One further problem is the fact-that the cooler and the magnet are only insufficiently decoupled concerning vibrations, such that the magnetic field of the magnet system may be influenced by the cryocooler. Moreover, NMR measurements cannot be performed during maintenance of the cooling system of this apparatus.
Some disadvantages of WO 03/023433 are eliminated in EP 1 560 035 (Oxford Instruments Superconductivity). The cooling device proposed therein has a better thermodynamic efficiency. However, there are still serious disadvantages. The cold head of the cryocooler is rigidly installed in the cryostat, such that vibrations and electromagnetic disturbances of the cold head are more or less directly transferred to the magnet cryostat. Conversely, the superconducting magnet in the magnet cryostat can influence the cryocooler. Maintenance or exchange of the cold head still involves considerable effort and costs, since operation of the entire magnet system must be stopped. Moreover, retrofitting of existing conventional magnet systems is difficult, which requires a completely new construction.
It is therefore the underlying purpose of the present invention to propose an NMR apparatus that eliminates the above-mentioned disadvantages and has a simple construction.